1. Field of the Invention
The present invention is directed to rigid, tough, fine-fibered non-woven polymeric, particularly polypropylene or polystyrene battery separators, and methods for producing them. These have been critically constructed to have the proper characteristics of porosity, stiffness, abrasion resistance, etc.
The invention involves a special melt-blowing process in which a fiber-forming polymer is extruded in molten form from orifices of a heated nozzle into a stream of hot gas to attenuate the molten resin as fibers which form a fiber stream, the fibers being collected on a receiver in the path of the fiber stream to form the non-woven mat which is formed into the battery separator of this invention.
2. Prior Art
Various melt-blowing processes have been described heretofore, earlier efforts including those of Hall (U.S. Pat. No. 2,374,540), Manning (U.S. Pat. Nos. 2,411,659; 2,411,660; and 2,437,263) and Marshall (U.S. Pat. No. 2,508,462 ). A melt-blowing process is disclosed in the article "Super-Fine Thermoplastics", by Van. A. Wente, in Industrial and Engineering Chemistry, Volume 48, No. 8 (1956), pages 1342-1346 and also in Naval Research Laboratory Report No. 111437, submitted Apr. 15, 1954, entitled "Manufacture of Super-Fine Organic Fibers". The Naval Research Laboratory process is further described in NRL Report 5265, dated Feb. 11, 1959, and entitled "An Improved Device for the Formation of Super-Fine, Thermoplastic Fibers". U.S. Pat. No. 3,532,800 to Wyly et al. discloses a use of the Naval Research Laboratory melt-blowing process. A melt spinning and blowing process is disclosed in British Pat. No. 1,055,187 and U.S. Pat. Nos. 3,379,811 and 3,502,763. As evidenced by these prior melt-blowing processes, it has been believed and taught that degradation of a fiber-forming thermoplastic polymer resin is to be avoided in a melt-blowing process.
Heretofore, non-woven mats made of essentially discontinuous fibers and produced by known melt-blowing processes have contained undesirable coarse "shot" or "beads" of material larger than about 0.3 millimeter in diameter. Moreover, prior melt-blowing processes operate at low and generally uneconomical resin flow rates of less than 1.0 gram per minute per resin outlet and experience difficulty in producing soft, fine, high quality mats that do not contain coarse shot. Also, earlier melt-blowing processes do not disclose how to produce mats substantially free of coarse shot from a fiber-forming thermoplastic polymer resin having a high intrinsic viscosity (1.4 or greater), particularly with C.sub.3 -C.sub.8 polyolefins, especially polypropylene. These polyolefins, which are conventionally produced in the presence of a heterogeneous solid catalyst, normally have very high intrinsic viscosities typically 2.2 to 4 and higher, corresponding to high viscosity average molecular weights of about 270,000 to about 550,000 and higher. Intrinsic viscosities as used herein are measured in decalin at 135.degree.C. The melt flow rates or melt indexes of these high intrinsic viscosity resins are quite low, typically about 5 to 0.5 and lower.
The commercially used battery separators are mainly of two types: (1) resin impregnated paper and (2) microporous rubber. The most commonly used material in auto batteries is resin impregnated paper. But, this material is rather brittle and is more susceptible to acid deterioration than is desirable.
The microporous rubber separators are also more brittle than is desired and are much more costly than the resin impregnated paper. The microporous rubber separators are used mostly in industrial battery applications.
It has been long recognized by the industry that battery separators made from polyolefins would possess the property of acid and alkaline resistance. But in spite of numerous and elaborate attempts, polyolefin battery separators possessing other essential properties such as stiffness, abrasion resistance, proper porosity, etc., have eluded the art. Examples of battery separators made from polyolefins, specifically polypropylene, described in the art are illustrated in the following patents: U.S. Pat. Nos. 3,002,040; 3,026,366; 3,045,058; 3,055,966; 3,092,438; 3,314,821; 3,351,495.
A particularly pertinent patent is U.S. Pat. No. 2,482,062. This teaches the preparation of battery separators from polystyrene fibers and other thermoplastics such as polyethylene having very small fiber sizes, i.e. diameters less than 5 microns with a degree of compaction between 30 and 60 percent. This patent recognizes the significance of small fiber size as one of the components determining proper porosity. Nevertheless, the battery separator of the present application is a decided improvement over the battery separator of the '062 patent because it embodies all of the necessary attributes for a commercial fibrous polymeric battery separator, not just small fiber size.
Although Wente shows a process for making mats of super-fine thermoplastic fibers, he has no concept of this invention which provides a battery separator possessing the controlling criteria of proper porosity, proper stiffness, proper abrasion resistance, etc.